Particulate matter filters are increasingly used in automotive emissions systems for reducing particulate concentrations in engine exhaust. When soot accumulates to a threshold level on the particulate filter, a filter regeneration process may be used to burn off the accumulated soot under controlled engine operating conditions. However, over time, such particulates filters can suffer irreversible decreases in trapping efficiencies as the filter develops cracks due to uncontrolled temperature excursion during the filter regeneration process. Losses in trapping efficiency of the particulate filter may result in increased particulate matter emissions well above the regulated limit.
Increasingly stringent particulate matter emissions standards and proposed government-mandated on-board diagnostic (OBD) requirements for monitoring the trapping efficiency of a particulate filter have stimulated much research into new techniques for monitoring particulate filter performance. One method includes determining a differential pressure across a particulate filter. If the differential pressure is less than a threshold differential pressure, then the particulate filter may be leaking. However, this method may not be suitable for detecting a failure of the filter due to interference effects from ash loading in the filter. Other methods to determine particulate filter leakage include utilizing a soot sensor, located downstream of a particulate filter, to monitor a soot load in exhaust flow and signaling when the soot load exceeds a soot threshold (e.g., the soot threshold may be based on a threshold amount of acceptable soot leakage based on particulate matter emissions).
However, the inventors herein have recognized potential issues with such systems. As one example, the soot sensor may have low sensitivity to leaked soot due to a relatively small portion of soot being deposited on the soot sensor. This may be due to an exhaust pipe geometry and/or poor mixing of the exhaust gas. Furthermore, large diesel particulates and/or water droplets may impinge onto surfaces of the soot sensor, altering the soot sensor reading.
In one example, the issues described above may be addressed by a method for diverting exhaust gas from an exhaust pipe to parallel first and second exhaust pathways outside the exhaust pipe. The second exhaust pathway includes a filter coupled to an electric circuit. The method further includes adjusting engine operation based on an estimated exhaust flow rate ratio of flow rates through the first and second exhaust pathways. The flow rates are based on pressure drops through respective venturi tubes of the first and second exhaust pathways. In this way, degradation of a particulate filter in an exhaust conduit may be accurately determined.
As one example, the parallel first and second exhaust pathway may be located downstream of the particulate filter in the exhaust conduit. As a number of regenerations of the particulate filter increases, the particulate filter may become degraded and an increasing amount of soot may travel downstream of the particulate filter. As a result, more soot may build up on the filter located in the second exhaust pathway and as a result, a backpressure may be introduced to the second exhaust pathway. The impedance of exhaust flow through the second exhaust pathway causes a ratio of flow through the first and second exhaust pathways to increase. Once the ratio reaches a threshold ratio, an electric circuit, electrically coupled to the filter in the second exhaust pathway, is used to regenerate the filter. Degradation of the particulate filter in the exhaust conduit may be indicated once a time interval between subsequent regenerations of the filter in the second exhaust pathway decreases to a time interval less than a threshold time interval.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.